Stickleback fish offer several "natural experiments" in evolution.
Courtesy David Kingsley, Stanford University.

The fish are found around the world, and closely-related species live side by side in some lakes and streams in North America. When Ice Age glaciers started retreating about 15,000 years ago, sticklebacks from the oceans colonized new lakes and rivers, adapting to fresh water and creating new species adapted to specific habitats.

These closely-related fish descended relatively recently from common ancestors. And that gives biologists a new way to look at evolution -- at how, exactly, organisms change and evolve.

It also supplies an excuse to go fishing, but that's another story.

More important, sticklebacks may illuminate the genetic mechanisms of differentiation -- the appearance of behavioral, ecological, physical and physiological changes that are passed down to progeny.

As a theory, evolution -- the change of organisms through natural selection -- is as well accepted as any in science. But specific questions of great importance remain. David Kingsley, professor of developmental biology at Stanford University, asks, "Does evolution occur through infinitesimally small genetic changes involving a very large number of genes, or does it occur through changes of large effect associated with a smaller number of genes?"

The plants and animals typically used in bio labs are little help, since they have been maintained by human selection under laboratory conditions, and thus may not be representative of natural conditions. "What we needed were two natural species that had diverged fairly recently, had distinct morphological differences, were fast-growing, and were easy to move into the laboratory for genetic experiments," said Kingsley.

Kingsley, an investigator at the Howard Hughes Medical Institute, is leading an effort to map the location of traits in sticklebacks, hoping to answer the big evolutionary question once and for all, with a well-studied but homely fish.

Sticking with the stickleback
The genetically diverse sticklebacks live in northern lakes, and some lakes house two species that, while reproductively isolated in the wild, can be bred in the lab. Kingsley and company made the first genome-wide linkage map of the threespine stickleback, bred two species, and watched how the genes controlled traits in the offspring.

The parent fish for an experiment just reported in Nature were adapted to deep or shallow parts of a single freshwater lake. The shallow-dwellers eat invertebrates, and the deeper-dwelling fish have more gill structures for trapping free-floating organisms. Because predators differ in the two environments, the fish also have different-sized protective spines.

It turned out that major traits, like the shape and number of spines and other bony structures, often resulted from a few major chromosome regions.

These results do not yet settle the age-old debate about whether evolution proceeds by "many small" or a "few large" genetic variations. The studies do, however, suggest that many new traits in sticklebacks may have a fairly simple genetic basis, and that the genetic changes that create new traits can now be studied in detail.

Spinal map
Kingsley found that the top upper spines were controlled by different genes, indicating that genetic control can be modular. That makes sense, Kingsley said, because to overcome the wide variety of challenges an organism faces in the environment, it may be best to change a single bone but not others.

However, some traits moved in tandem, as would be expected for traits that have common functions. For example, the longer spine on the top, and the single spine on the bottom, were both controlled by a single gene -- a finding that reflected field observations of simultaneous changes in both spines.

Overall, the research, and further studies focusing on other "evolutionary experiments" provided by other sticklebacks, should shed light on how evolution adapts species to the many environments found in post-glacial lakes, not to mention different environments around the globe.

Not bad for a fish that's essentially inedible.

- David Tenenbaum

BIBLIOGRAPHY
The Genetic Architecture of Divergence Between Three-Spine Stickleback Species, Catherine Peichel et al, Nature, 20 Dec. 2001, pp. 901-5.

More on the story.